He was one the headliners on the lineup -- performing alongside acts like A Boogie wit da Hoodie, Ally Brooke, YK Osiris, Big Freedia and others -- but because he couldn't make it ... DMX filled in for him last minute. Not a bad replacement.

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Wiz took to social media Saturday to apologize to fans who'd purchased tickets in hopes of seeing him hit the stage, but promised he'd make it up to them in the future when he was healthy. He also made sure to thank DMX for stepping in on short notice.

If the MMA thing is catching you off guard, it shouldn't. He's been at this for at least a couple years now ... and he's absolutely taking training seriously. Just ask FOX's Jay Glazer.

What's crazy about this unfortunate setback for Wiz is that he appears to wear protective gear on his legs and ankles when he trains -- at least he did in this video here. He must've been kicking some serious ass to splinter a bone down there ... yowza. BTW, it should be noted ... we're told Wiz was NOT training with Jay in this instance.

Anyway, for all the fans who are upset, cut the dude a break -- a metaphorical one, of course.